Many displays incorporate a transmissive display panel and a backlight for illuminating the display panel. FIG. 1(a) is a block schematic view of such a display 1. The display 1 comprises a transmissive display panel 2, for example a transmissive liquid crystal display panel, a light source 4 and a light guide 5. The light source 4 and the light guide 5 together comprise a backlight 3. In use, the display panel 2 is illuminated by the backlight 3 such that an image displayed on the display panel 2 is visible to an observer located on the opposite side of the display panel to the backlight 3. Light from the backlight is designed to pass though elements of an LCD display panel such as polarising filters, TFT layers, liquid crystals film, colour filters etc. finally reaching the observer. Such a display is described in more detail in, for example “Liquid Crystal Flat Panel Displays” by William O'Mara (1993).
Transflective displays are also known. These have a structure generally similar to the display of FIG. 1 but incorporate a partially-reflecting layer disposed behind the image display layer of the display panel 2. (By “behind” the image display layer is meant that the partially-reflecting layer is on the opposite side of the image display layer to the observer.) The display panel 2 may be illuminated by the backlight 3 in low ambient lighting conditions or by reflected ambient light in bright ambient lighting conditions.
Front-light displays are also known, which incorporate a reflective layer disposed behind the image display layer of a display panel. The display panel may be illuminated by a frontlight in low ambient lighting conditions or by reflected ambient light in bright ambient lighting conditions.
Where an image display layer of a display is illuminated by a backlight, it is important that the backlight provides a bright image, so that the image is visible to an observer in any ambient lighting conditions. It is important that the backlight provides a high colour gamut, so that a full-colour image is correctly reproduced.
FIG. 1(b) shows the CIE 1931 colour space chromaticity diagram. The enclosed area 16 in this diagram represents all wavelengths visible to a human (the “gamut of human vision”). The outer curved portion 15 is the spectral (monochromatic) locus, with the numbers indicating wavelengths shown in nanometers. A light source may be represented by a point in the colour space, with a truly monochromatic light source being represented by a point on the spectral locus.
A backlight may consist of three light sources of different emission wavelengths, for example light sources that emit in the red, green and blue regions of the spectrum. All colours that may be formed using three light sources are represented by the interior of the triangle defined on the CIE chromaticity diagram by the three points corresponding to the sources. FIG. 1(b) shows a triangle 17 corresponding to the colour space defined by the US National Television System Committee.
In assessing the colour gamut of a backlight for a display, one measure is the “NTSC ratio”. This may be defined as:
            Area      ⁢                          ⁢      of      ⁢                          ⁢      colour      ⁢                          ⁢      space      ⁢                          ⁢      of      ⁢                          ⁢      backlight      ⁢                          ⁢      on      ⁢                          ⁢      CIE      ⁢                          ⁢      diagram              Area      ⁢                          ⁢      of      ⁢                          ⁢      NTSC      ⁢                          ⁢      colour      ⁢                          ⁢      space      ⁢                          ⁢      on      ⁢                          ⁢      CIE      ⁢                          ⁢      diagram        =      NTSC    ⁢                  ⁢    ratio  
Recently manufacturers of Liquid Crystals Display manufacturers have increasingly been using LEDs (light-emitting diodes) as light sources in backlights for displays in mobile phones, PC monitors and televisions. LEDs potentially have several advantages over conventional light sources, such as long lifetime and high efficiency.
White light can be produced using LEDs by mixing red, blue and green light from an array of separate blue, red and green LEDs as disclosed in, for example, U.S. Pat. No. 6,608,614 and U.S. Pat. No. 6,768,525. A high NTSC ratio can be achieved using individual, epitaxially grown, blue, red and green LEDs. However, this method is expensive and has potential problems with colour mixing and complexity of the required electronics circuitry for driving the LEDs. A further problem is that the relative degradation of the three coloured LEDs may be different, leading to colour shifts during extended operation. Yet another problem is that fabricating efficient green LEDs still remains a challenge. Nevertheless, this approach provides the highest values so far reported for the NTSC ratio.
An alternative approach is to use an monochromatic LED as a primary light source to illuminate a medium, such as a phosphor, that converts all or part of the light from the LED to light of another wavelength so that a white light output is obtained. For example, use of an blue LED to illuminate a yellow phosphor leads to part of the output light from the LED being absorbed and re-emitted by the phosphor in the yellow region of the spectrum, to produce a white light. (The term “blue LED”, for example, as used herein refers to an LED that emits light in the blue region of the spectrum; similarly, the term “yellow phosphor”, for example, as used herein refers to a phosphor that, when illuminated by a light of a suitable wavelength, re-emits light in the yellow region of the spectrum.)
Japanese Journal of Applied Physics, Vol. 44, No. 21, pp. L 649-L 651 (2005) reports a phosphor-converted white LED (WLED). A WLED consists of a primary blue or ultra-violet (UV) LED illuminating a white, yellow or red/green fluorescent phosphor layer which down-converts all or part of the primary blue/UV excitation light to emit white light.
One disadvantage of this method, when applied to a backlight for a display device, is that phosphor converted LEDs generally have a broad emission peak, as shown in the left hand part of FIG. 2(a). When used as a backlight for a display having conventional colour filters with the typical absorption characteristics shown in the central part of FIG. 2(a), an NTSC ratio of only around 65% is possible.
The emission spectrum of FIG. 2(a) is for YAG-based phosphor materials. Several YAG-based fluorescent materials have been used as phosphors for phosphor-converted LEDs. Example of these materials are disclosed in U.S. Pat. No. 5,813,753 and U.S. Pat. No. 5,998,925.
A higher NTSC ratio of 104.2% can be achieved, when using conventional phosphor WLEDs, by using narrow-band colour filters in the display panel. This is illustrated in FIG. 2(b), in which the left hand figure shows the emission spectrum of the phosphors (which is generally similar to the phosphor emission spectrum of FIG. 2(a)), and the central figure shows the characteristics of the colour filters of the liquid crystal panel. However, modelling results show that the overall brightness is greatly reduced by the use of narrow-band colour filters in the display panel—compared to FIG. 2(a), the efficiency (defined as the % of light output from the backlight that is not absorbed within the display panel) is reduced by 30%. In order to compensate for this reduction in efficiency the backlight must be driven to give a greater output power, and this will shorten the lifetime of the backlight and increase power consumption.
U.S. Pat. No. 6,809,781 and US 2004/0056990 describe using selection of conventional phosphor blends in a transparent matrix, which emit strongly in the wavelength range specific to the LCD colour filters in order to enhance the brightness of the LCD.
U.S. Pat. No. 6,637,905 describes a backlight using such conventional phosphors in which phosphors are placed remotely and irradiated by a primary source. The heating in the phosphors is largely by the remote location of the phosphors; hence this maintains a uniform emission from the device over a longer period of time. These types of WLEDs attain a quasi-white emission but lack strong contributions in the red region of the spectrum.
US 2004/0207313 describes white LEDs in which white light is generated from various combinations of green phosphors, red phosphors, blue LEDs and red LEDs. In this invention the red LEDs compensate the lack of red light and thus a better colour characteristic is attained.
GB 2 425 393, published after the priority date of this application, relates to a display panel which can display both a “primary image” and a “secondary image” such as a company logo. A region of wavelength converting material is provided in the display panel, and the display panel is illuminated by an array of light sources that includes first light sources for providing the primary image and second light sources for illuminating the wavelength conversion material. The secondary image may be projected through a filter layer which blocks the light that is used to excite the wavelength converting material.